In analyzing samples containing gases, it is often necessary or desirable to ascertain the partial pressure of various gases contained within the sample. A common example is determining the partial pressures of gases in blood, such as carbon dioxide and oxygen. In addition, pH value, the concentration of certain ions and temperature of the blood may need to be determined.
One prior art technique for determining the partial pressure of carbon dioxide employs an electrode with an indicator layer sandwiched between the electrode and a semi-permeable membrane. When the membrane is placed in contact with the blood, the membrane transmits the carbon dioxide to the indicator and essentially excludes the liquid component of the blood. The indicator, which is retained in a nylon mesh, responds to the carbon dioxide, and this response can be measured electrically to provide an indication of the partial pressure of carbon dioxide in the blood.
An optical technique for measuring the partial pressure of blood gases is disclosed in Lubbers et al U.S. Pat. No. 4,003,707. The Lubbers patent discloses an optode which comprises an indicator confined between a semi-permeable membrane and a light-transmitting member of glass or other material. Light is directed through the light-transmitting member to the indicator, and the indicator provides a fluorescent output signal. The membrane transmits the gas of interest, and the indicator responds to the partial pressure of such gas to vary the output signal.
The optical technique disclosed in the Lubbers et al patent has the advantage of avoiding the electrical apparatus necessary for utilizing the electrode type sensor. However, Lubbers et al's optode in the form shown in the patent would be difficult and costly to manufacture in quantity. In addition, if the membrane develops a slight crack during use or storage, the usefulness of the sensor is lost.